1. The Field of the Invention
The present invention relates to wireless radio receiver technology and, more specifically, to improved circuits and methods for the direct conversion of radio frequency modulated signals to baseband signals without requiring conversion through an intermediate frequency.
2. The Prior State of the Art
Electrical signals have proven to be an effective means of conveying data from one location to another. The further a signal is transmitted, however, the greater the decay in the signal and the greater the chance for irreversible loss in the data represented by the signal. In order to guard against this signal decay, the core electrical signal that represents the data (i.e., the baseband signal) may be modulated or superimposed on a carrier wave in the Radio Frequency (RF) frequency spectrum.
In order to properly interpret the signal, conventional RF receivers demodulate the baseband signal from the received signal. The data represented by the extracted baseband signal may then be interpreted by other downstream circuitry.
In order to perform this demodulation, typical receivers include circuitry which first converts the received radio frequency modulated signal into an intermediate frequency (xe2x80x9cIFxe2x80x9d) signal. This intermediate frequency signal is then converted into the baseband signal for further data processing. Receiver architectures that convert through the intermediate frequency are often called xe2x80x9cheterodynexe2x80x9d receiver architectures. Naturally, circuit elements (called xe2x80x9cIF componentsxe2x80x9d) are required in order to deal with the intermediate conversion to and from the intermediate frequency.
It is desirable to reduce the cost, size, and power consumption of a particular receiver architecture design for strategic marketing of the receiver. This is particularly true of wireless RF receivers since those receivers are often portable and run on battery power.
One technology developed in order to reduce RF receiver cost, size, and power consumption is called xe2x80x9cdirect conversion.xe2x80x9d Direct conversion refers to the direct conversion of RF modulated signals into corresponding baseband signals without requiring conversion through the intermediate frequency. Such direct conversion receiver architectures are often called xe2x80x9czero-IF,xe2x80x9d xe2x80x9csynchrodyne,xe2x80x9d or xe2x80x9chomodynexe2x80x9d receiver architectures.
FIG. 1 illustrates a conventional direct conversion circuit 100 in accordance with the prior art. The circuit 100 includes an antenna 101 which receives the RF modulated signal. The antenna then provides the received signal to an amplifier 102 which amplifies the signal for further processing. The amplifier 102 may be, for example, an RF low noise amplifier.
The amplified signal is then split into two branches, an xe2x80x9cin-phasexe2x80x9d branch 110, and a xe2x80x9cquadrature-phasexe2x80x9d branch 120. Each branch includes a mixer that initially receives the amplified signal. For instance, the in-phase branch 110 includes an in-phase mixer 111, and the quadrature-phase branch 120 includes a quadrature-phase mixer 121. A local oscillator 130 provides a sine or square wave signal as a control signal to each of the mixers. Each mixer is configured to nonlinearly process the amplified signal and control signal, resulting in output signal components at frequencies equal to the sum and difference of amplified signal and control signal frequencies, plus higher-order components at other frequencies. The circuit includes a 90-degree phase shifter 131 which causes the control signal for the quadrature-phase mixer 121 to be 90 degrees out of phase with the control signal for the in-phase mixer 111.
The signal from the in-phase mixer 111 is then passed through a low pass filter 112 to a baseband amplifier 113 to complete the extraction of the baseband (difference frequency) signal from the received signal as far as the in-phase branch 110 is concerned. Likewise, the signal from the quadrature-phase mixer 121 is passed through a low pass filter 122 to a baseband amplifier 123 to complete the extraction of the baseband (difference frequency) signal as far as the quadrature-phase branch is concerned. The quadrature baseband signals are then processed by signal processing circuitry 150.
The direct conversion circuit of FIG. 1 does not convert through an intermediate frequency and thus there are no IF components needed to deal with an intermediate conversion. Consequently, the direct conversion circuit of FIG. 1 is smaller, and requires less power than conventional heterodyne receiver architectures, which perform intermediate conversion.
However, there are some performance issues for the direct conversion circuit of FIG. 1 that limit its practical implementation. First, there is often local oscillator leakage to the antenna 101, to the amplifier 102 input and to the mixer inputs. This results in local oscillator Direct Current (DC) self-mixing products that can overpower and degrade the baseband signals. Second, the antenna may radiate the local oscillator leakage causing an interference problem for other nearby receivers. This may also cause a time-varying self-mixing product due to radiated RF leakage reflecting off nearby objects, possible in motion, and being received back at the same antenna, then adding to the leakage component present at the antenna terminal. Third, there is a lack of RF selectivity, combined with amplifier and mixer limited dynamic range, resulting in direct AM detection of high level, in-band or adjacent channel interference. The net result of these performance degradations is that there may be some loss of data in the baseband signals and some interference with the operation of nearby receivers.
What is therefore desired are circuits and methods for direct conversion of RF modulated signals directly into baseband signals without intermediate conversion through and intermediate frequency while reducing the above-described problems related to oscillator leakage, interference with surrounding antennas, self-mixing products at the antenna, low RF selectivity, and limited dynamic range.
A direct conversion receiver converts a radio frequency modulated (RF) signal into a corresponding baseband signal without requiring conversion through an intermediate frequency. The direct conversion receiver abates local oscillator leakage, increases dynamic range and increases RF selectivity as compared to conventional direct conversion circuits.
After being received and amplified as necessary, the RF signal is provided to two branches of the direct conversion circuit, an xe2x80x9cin-phasexe2x80x9d branch and a xe2x80x9cquadrature-phasexe2x80x9d branch.xe2x80x9d However, instead of the conventional one mixer per branch, each branch includes two mixers. These four mixers periodically pass on the RF signal if the corresponding control signal provided to the mixer is high. A local oscillator provides the controls signals in the form of binary waves which have the same period, nominally, as the carrier period of the RF signal. Each mixer is provided with a primary binary control signal having a duty cycle of approximately 25 percent in the logic xe2x80x9chighxe2x80x9d state and a corresponding binary complement signal having a duty cycle of approximately 75 percent in the logic xe2x80x9chighxe2x80x9d state. The quarter-period xe2x80x9chighxe2x80x9d states of the primary control signals are time-shifted, from one control signal to the next, to produce quadrature-phased, primary binary control signals with relative phases of approximately 0, 90, 180, and 270 degrees.
As for the in-phase branch, a first mixer has an output terminal coupled to the positive terminal of the operational amplifier. This first mixer is provided with a corresponding first primary binary control signal. A second mixer in the in-phase branch has an output terminal that is coupled to the negative terminal of the operational amplifier. This second mixer is provided with a second primary control signal that is 180 degrees out of phase as compared to the first primary control signal. An operational amplifier receives the signal passed by the two mixers in the in-phase branch and generates a signal that represents the difference between the two signals provided by the mixers. The resulting signal is then passed through a low pass filter to generate a first baseband signal.
As for the quadrature-phase branch, a first mixer has an output terminal coupled to the positive terminal of another operational amplifier. This first mixer is provided with a corresponding primary binary control signal that is 90 degrees out of phase. A second mixer in the quadrature-phase branch has an output terminal that is coupled to the negative terminal of the operational amplifier. This second mixer is provided with a primary control signal that is 270 degrees out of phase. This operational amplifier receives the signal passed by the two mixers in the quadrature-phase branch and generates a signal that represents the difference between the two signals provided by the mixers. The resulting signal is then passed through another low pass filter to generate a second baseband signal. The two quadrature baseband signals are then provided to signal processing circuitry for further processing.
The direct conversion circuit described herein has many advantages over conventional direct conversion circuits. Specifically, the signals provided to each mixer are balanced complementary signals. Thus, any leakage from a pair of complementary signals tends to cancel each other out. Also, the superposition of the two primary switching control signals provided to each branch is a square wave at twice the RF carrier frequency. Likewise, the superposition of the two complementary switching control signals provided to each branch is also a square wave at twice the RF carrier frequency.
Also, coupling of the four switching control signal waveforms in the two switching mixers of each branch via a capacitive leakage path to a distant node results in an xe2x80x9cimpulse trainxe2x80x9d at that node which is balanced. Impulse trains occur at the leading and lagging edges of the control signals thus forming a positive-going impulse train at four times the receive frequency and a coincident negative-going impulse train at four times the receive frequency. Thus, coupling along balanced leakage paths tend to cancel each other.
In addition, the superposition of all eight control signals is a balanced waveform that contains no spectral components at the receive frequency. These effects combine to reduce the local oscillator leakage to surrounding components including the antenna.
Furthermore, the circuit results in reduced AM detection in the mixers due to the use of switches for mixers, as opposed to xe2x80x9cactivexe2x80x9d mixer implementations such as Gilbert cells. Switching mixers can be implemented with active semiconductor switching devices where the degree of non-linearity incurred across the switch itself in the xe2x80x9conxe2x80x9d state is less than with Gilbert cells or semiconductor diode ring devices. Therefore, larger second order and third order intercept values can be realized.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.